1. Field of the Invention
This invention relates to weather resistant impact modifiers for thermoplastic resins which can impart high impact resistance and good processability to the resins, and thermoplastic composition comprising a thermoplastic resin and the modifier.
2. Description of the Prior Art
Thermoplastic resins, particularly vinyl chloride resins (hereinafter abbreviated as "PVC") have been widely employed as general-purpose resins but their mechanical properties are not necessarily satisfactory. More specifically, PVC is poor in impact strength, particularly in notched impact strength. In order to improve the impact strength, there have been proposed a variety of modifiers.
Among these proposals, there are known some effective methods including a method wherein so-called a MBS resin obtained by graft polymerization of an alkyl methacrylate and an aromatic vinyl compound to a conjugated diene elastomers is blended with PVC, and a method wherein an ABS resin obtained by graft polymerization of a vinyl cyanide and an aromatic vinyl compound to a conjugated diene elastomer is blended with PVC. However, since the MBS resin and ABS resin contain large amounts of double bonds in the main chain of the elastomer components, they will cause a choking phenomenon and lowering of impact strength when used over a long term in outdoor environment. Thus, these resins are not suited for outdoor applications.
A number of resin compositions having excellent weather resistance have also been proposed wherein a copolymer obtained by graft polymerization of an alkyl methacrylate, an aromatic vinyl compound and a vinyl cyanide to a saturated polyalkyl acrylate elastomer is blended with PVC. Although these compositions exhibit good impact strength at ambient temperatures, the impact strength at low temperatures is not always satisfactory because of the relatively high Tg value of the polyalkyl acrylate.
Moreover, U.S. Pat. No. 4,393,172 has proposed a graft copolymer using a diene rubber as a part of a core component, but the effect on improvement in the impact strength is not satisfactory.
When blended with thermoplastic resins, conventional modifiers for thermoplastic resins have disadvantages that the weather resistance of the resins is worsened and that an effect of the modifiers on imparting impact strength to the resins is not satisfactory. Thus, modifiers capable of imparting excellent impact strength, weather resistance and processability to the resins have not been obtained yet.
As the results of intensive studies for obtaining a modifier, which has excellent weather resistance and deep effect on imparting impact resistance to the resins not only at ambient temperatures but also at low temperatures, it has been found that a graft copolymer, which is obtained by graft polymerization by two stages of monomers having specific compositions to an elastomer which has a two-layered structure and is obtained by polymerizing an acrylic ester or a monomer mixture mainly composed of the acrylic ester in the presence of a butadiene rubber having specified properties, is useful as a modifier having intended modification effects.